This flag is the WebAssembly native heap equivalent to
FLAG_write_protect_code_memory.

R=mstarzinger@chromium.org

Bug: v8:7454
Change-Id: Id4f671af2e8676d08599c8c30ce03b00e9d33780
Reviewed-on: https://chromium-review.googlesource.com/924071
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51330}